A system for manipulating phasic aortic blood pressure in closed chest experimental dogs has been under development. An intra-aortic balloon residing in the descending aorta and controlled by feedback from a catheter tip pressure transducer is phasically inflated or deflated by a piston type actuator to vary the pressure to conform with a predetermined desired pressure waveform. While still under servo control, the balloon is reset (either filled or deflated) every few beats due to the action of a roller pump which bypasses the systemic circulation in order to withdraw (or infuse) blood into the aorta. In this way it was hoped to control both the average pressure level and the exact waveform so that the effects on the myocardium of various drug interventions could be evaluated independent of their effects on systemic blood pressure. During the past year the system was evaluated both in dogs, and in an elaborate in vitro model of the aorta and systemic circulation. A number of problems were identified which could only be solved by placing constraints on the system which ultimately proved to be mutually incompatible - resulting in termination of the overall effort. The fundamental problem which could not be overcome arose from the large phase shift and lack of attenuation with increasing frequency between actuator position and aortic pressure which arises due to the distributed compliance and low wave speed of the aorta. This undesirable open loop characteristic precluded attempts to design suitable servo compensation, resulting in phasic waveforms that did not faithfully reproduce the desired waveform. This situation could be partially mitigated only by making the balloon small - which resulted in a need to reset the balloon too often for the system to function properly.